1 Field of the Invention
The present invention relates to a transmission system for use in a work vehicle, and more particularly to an art of controlling a hydraulic clutch included in the transmission system.
2 Description of the Prior Art
In a work vehicle such as an agricultural tractor, the transmission system includes a gear type change-speed unit and a hydraulic clutch related to the change-speed unit. For effecting a change-speed operation, the hydraulic clutch is once disengaged. Then, the clutch is again engaged for power transmission upon completion of the change-speed operation. If this clutch engagement is effected rather hastily, this results in an operational shock adversely influencing the travelling vehicle. Reversely, if the clutch engagement is effected slowly for avoiding the shock, this results in an undesirable delay in the vehicle travelling control operation.
In view of this problem, it is known, as shock preventing means, to provide a control unit operable to adjust the amount of work oil supplied to the hydraulic clutch based on a detected value concerning a rotational velocity of the transmission system (Japanese patent laid open under Showa No. 62-215155). It is also known to provide a work vehicle body with an acceleration sensor operable to generate, when a change-speed operation is effected, an acceleration indicative signal to be fed back to a control unit of the hydraulic clutch such that the control unit adjusts the amount of oil to the clutch based on the feedback signal (Japanese patent laid open under Showa No. 62-231841). There is still further known art (no particular reference cited) in which the oil supply to the hydraulic clutch needed with a change-speed operation is adjusted in accordance with predetermined optimum oil supply characterisitcs (i.e. for most speedily engaging the clutch without causing the engagement shock).
Considering the causes of clutch engagement shock, with supply of work oil, the hydraulic clutch reaches its full-clutched condition after a half-clutched condition. An engagement shock occurs if the half-clutched condition has been maintained only for a very brief period of time or the half-clutched condition has been maintained in an unstable manner. On the other hand, for a speedy clutch engagement, clutching plates mechanically separated from each other under a disengaged condition of the clutch should come into mutual contact as quickly as possible for providing the half-clutched condition. In view of these facts, if the hydraulic clutch is controlled according to a rotational velocity of transmission system or to an acceleration rate of the travelling system as suggested by the above-described prior references, under a relatively large driving load, the transmission system cannot be driven with such half-clutched clutched condition but can be driven only with complete clutch engagement, i.e. full-clutched condition. Accordingly, in such situation, these prior methods do not function properly for avoiding the engagement shock. Moreover, if the hydraulic clutch is controlled by such predetermined, i.e. fixed characteristics, the control will fail to cope with load variations as developed when the change-speed operation is effected between various number of steps or when the vehicle encouters changes in such external factors such as a slope of travelling passage. Further, determination of such control characterisitcs per se will be difficult if they are to consider numerous factors such as oil viscosity subject to temperature variations and an amount of oil remaining at the clutch.
The primary object of the present invention is to provide a work vehicle which may constantly travel at an optimum operational condition with a smooth and speedy engagement of a hydraulic clutch regardless of possible changes in the driving load or external factors.
In order to accomplish the above-noted object, a work vehicle according to the present invention comprises: a hydraulic clutch; a torque sensor for detecting an amount of transmission torque of a travelling transmission system; a control valve for adjusting an amount of work oil to be fed to the hydraulic clutch; valve control means for controlling operations of the control valve in accordance with a predetermined control scheme designed for engaging hydraulic clutch, the control scheme including, a first step of opening the control valve to a first predetermined degree, a second step of temporarily closing the control valve to a second predetermined degree lower than the first predetermined degree and then opening the same again up to a full-open degree, and a third step of maintaining the opened condition of the control valve achieved by the second step; wherein the second step is initiated when the torque sensor has detected a rise in the transmission torque.
According to this construction, the hydraulic clutch has its oil supply amount controlled through the control valve which in turn is controlled by the valve control means. More particularly, there is provided the torque sensor to the travelling transmission system of the vehicle so as to detect a transmission torque being developed in the system. On the other hand, the valve control means effects its valve control scheme in three steps. In the first step, the control valve is opened to the first predetermined degree whereby currently separated friction (clutching) plates inside the clutch casing start contacting each other for providing a half-clutched condition. In this step, the oil is supplied quickly to the clutch through the control valve until the half-clutched condition is achieved. Then, in the second step, the control valve is temporarily closed down to the second predetermined degree so as to maintained the half-clutched condition achieved in the first step. Further in this second step, the temporarily closed control valve is again gradually opened to its maximum degree of providing a full-clutched condition. In the third step thereafter, this full-clutched condition is effectively maintained. Then, in the present invention, the shift timing between the first control step and the second control step coincides with the transmission torque rise detection made by the torque sensor. That is to say, a relatively large amount of oil is supplied to the clutch until there develops a rise in the transmission torque. Then, upon detection of torque rise; namely, upon initiation of power transmission under the half-clutched condition, the oil supply is first reduced temporarily and then increased gradually up to the maximum value corresponding to the full-open valve condition, i.e. full-clutched condition.
Accordingly, even if the vehicle driving load changes to disable effective power transmission to the travelling system, the hydraulic clutch may be speedily and shocklessly engaged through the proper detection of half-clutched condition.
Consequently, through such simple improvement including the torque sensor for the transmission system and the control scheme executed based on a detection of the torque sensor, the present invention has fully achieved the intended work vehicle which may always travel at the optimum operational condition with the speedy yet shockless engagement of the hydraulic clutch.
According to one preferred embodidment of the present invention, the re-opening operation of the control valve by the second step is executed based on a detection by the torque sensor so as to permit a transmission torque value of the travelling transmission system to follow a predetermined rising tendency. With this arrangement, the re-opening operation of the control valve in the second step is also effected based on the detection by the torque sensor. In comparison with the aforementioned rotational-velocity-based or acceleration-based control scheme of the prior art, this arrangement is advantageous for providing further improved half-clutching control in terms of engagement accuracy and speed and also power transmission efficiency. That is, this additional arrangement permits the half-clutching control to respond even more quickly to the driving load change. Accordingly, the work vehicle provided with this arrangement will benefit from further improved clutch control free from operational delay.
According to a further embodiment of the present invention, the first predetermined valve opening degree of the first step is set lower than the full-open degree maintained by the third step. If the valve were opened almost up to the maximum, i.e. full-open condition in the first step, this would deteriorate the shock-preventing effect of the temporary and partial valve closure effected in the second step by developing an unstable oil pressure condition in the clutch. In such unstable oil pressure condition, it has been experimentally learned that the clutch fails to effectively maintain the desired half-clutched condition and tends to become engaged to a degree exceeding the desired half-clutched condition thereby causing an engagement shock in spite of the temporary oil pressure reduction. For this reason, if the first predetermined valve opening degree is set lower than the full-open valve condition thereby providing a signal corresponding to a pressure close to the target pressure to be achieved by the second step, the oil pressure condition inside the clutch casing immediately after the shifting from the first to second step may be quickly stabilized thereby achieving quick and shockless clutch engagement .